The present invention relates to a training procedure performed in a parallel channel transmission system prior to the transmission of message data and, more particulrly, to a method of and apparatus for determining a time origin (starting point) of a timer which is used in a modem.
Training in data transmission serves various functions at each of transmitting and receiver units: at a receiver unit, converging automatic gain controller (AGC), setting up phases of a carrier and a timing clock, and converging an automatic equalizer and, at a transmitting unit, sequentially transmitting signals formatted each in a special pattern for fulfilling such functions at the receive unit over a predetermined period of time. The receiver unit includes a timer for recognizing transition of the predetermined period of time, and the timer requires a definite and accurate time origin. Should the time origin fail to be set with accuracy, training would be disabled from establishing phases of the signals or from converging the automatic equalizer at the receiver unit, thereby preventing initiation of data transmission.
In a single channel transmission system, it has been customary to determine a time origin of a timer taking advantage of the fact that signals formatted in special patterns as described have different spectra in a single channel. Specifically, use is made of a filter for identifying a spectrum particular to a special pattern, so that a time origin of a timer can be determined by detecting transition of the spectrum. However, in a parallel channel transmission system such as disclosed in U.S. Pat. No. 4,300,229, the band assigned to each channel is so narrow that direct application of the above-described method results in the need for filters for spectrum discrimination the bands of which are far narrower than the transmission band. Such filters are not only difficult to design but also cause of an increase in costs.